Projectors are one type of projection-type display devices that modulate emitted light from a light source in response to image information and magnify and project the optical image. Recently, projectors, together with personal computers, have been widely used for presentations at conferences and the like. Further, projectors also have been used in ordinary households for a home theater application.
For example, high-pressure mercury-vapor lamps, which are closer to a point source and have high color rendering, have been utilized in projectors as their light sources. Specifically, such a high-pressure mercury-vapor lamp is provided with an arc tube filled with mercury as a light-emitting material in an amount of, for example, 200 mg/cm3 or more and a pair of electrodes made of tungsten is disposed in the arc tube to substantially face each other. In addition to mercury, the arc tube is also filled with a halogen material. Consequently, a so-called halogen cycle effect is used to prevent tungsten as a constituent material of the electrodes from being scattered during lighting and adhering to the inner surface of the arc tube to cause blackening of the inner surface.
On one hand, the use of the halogen cycle effect allows the prevention of blackening of the arc tube inner surface. But on the other hand, it has been known that tungsten scattered from the electrodes returns and adheres to the electrodes, forming protrusions as it is being accumulated. When such protrusions are formed properly on the tips of the electrodes, the tips form arc bright spots, thereby allowing a stable arc and the prevention of occurrence of a so-called flicker resulting from movements of the arc cathode spots.
For this reason, it has been proposed conventionally to change the frequency of an alternating current supplied to a high-pressure mercury-vapor lamp in order to grow and maintain the protrusions properly for stabilizing an arc on the tips of the electrodes (Patent document 1, for example).
Further, as a method of changing the frequency of an alternating current, a method of changing the frequency constantly without depending on operation data of a lamp, in other words frequency modulation control, has been known (Patent document 2, for example). Unlike so-called restorative control where the control is performed after the protrusions become deformed, in this method, control is performed before the protrusions become deformed. Therefore, this method is advantageous in that the initial shape of the protrusions can be stably maintained.
A high-pressure mercury-vapor lamp of such a type is started and is lighted as follows. That is, for example, after a high voltage has been applied and a discharge has started (after an electrical breakdown between the electrodes), a starting operation is performed to stabilize the discharge. During the starting operation, constant current control using a high-frequency current selected from the range of 10 kHz to 500 kHz, for example, is performed for a certain period of time. After the completion of the starting operation, constant current control using a substantially square wave current with a frequency selected from the range of 20 Hz to 1000 Hz, for example, is performed. And afterwards, lighting is shifted to constant power lighting.    Patent document 1: JP 2001-312997 A    Patent document 2: Japanese Patent No. 3851343 A